Historically, sash type windows have displayed two major disadvantages, the first being the difficulty in obtaining a satisfactory seal between adjacent sash rails and the second being the difficulty in gaining access to the outer facing surface of the lower sash to facilitate its cleaning. A partial solution to these problems is disclosed in U.S. Pat. No. 5,090,750. In said patent a locking mechanism mounted on the transverse rail of one sash comprises a pair of sash clamps which can be brought into engagement and disengagement with corresponding receiving elements on the adjacent rail of the other sash. In addition, locking bolts mounted on the transverse rail of one sash can be brought into engagement with a sidewall of a guide groove in the window frame. By the provision of a complex series of linkages, a single handle can control the relative positioning of the sash clamps and the locking bolts between three distinct positions, i.e. a locked position in which the sash clamps engage the receiving elements and the locking bolts engage the sidewall of the guide groove, an unlocked position in which the sash clamps and the locking bolts are disengaged from the receiving elements and the sidewall respectively, and a release position in which the locking bolts are retracted from the groove to thereby allow the lower sash to be swung away from the window frame.
Whilst the above-described arrangement offers certain advantages over prior locking mechanisms, its complex series of linkages requires that the operating handle be both rotatably and vertically displaceable to effect the required operating sequence.